Methods of using carbon quantum dots to enhance productivity of fluids from wells

ABSTRACT

Methods of determining a pH of a wellbore fluid within a wellbore in communication with a subterranean formation comprise introducing carbon quantum dots into a wellbore fluid, exposing the wellbore fluid to radiation from an electromagnetic radiation source, and measuring at least one fluorescence property of the carbon quantum dots within the wellbore fluid to determine a pH of the wellbore fluid. Related methods of determining a pH of a fluid within a wellbore extending through a subterranean formation are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/172,335, filed Jun. 3, 2016 and entitled “METHODS OF USING CARBONQUANTUM DOTS TO ENHANCE PRODUCTIVITY OF FLUIDS FROM WELLS,” which is acontinuation-in-part application of U.S. patent application Ser. No.14/739,629, filed Jun. 15, 2015, now U.S. Pat. No. 9,715,036, issuedJul. 25, 2017, and entitled “WELLBORES INCLUDING CARBON QUANTUM DOTS,AND METHODS OF FORMING CARBON QUANTUM DOTS,” the disclosure of each ofwhich is incorporated herein it its entirety by this reference.

TECHNICAL FIELD

Embodiments of the disclosure relate generally to methods of formingcarbon quantum dots, methods and systems of using the carbon quantumdots to determine at least one property within subterranean formationsand methods of using the carbon quantum dots to enhance the productivityof hydrocarbon-containing fluids from the subterranean formations.

BACKGROUND

During formation and operation of a wellbore, it may be desirable tomeasure at least one property within a subterranean formation throughwhich the wellbore extends. For example, a high pH may be a precursor ofscale build-up and a low pH may be a precursor to corrosion of wellboreequipment. Thus, the pH of a formation fluid is conventionally monitoredto aid in reducing scale build-up and potential corrosion of thewellbore equipment.

Conventionally, the pH of the formation fluid is determined by obtaininga sample of the formation fluid and analyzing the sample in alaboratory. However, as the formation fluid is brought from formationconditions (e.g., high-temperature high-pressure conditions), acid gasesand salts may come out of solution, irreversibly changing the pH of thesample. Thus the analyzed sample may not be an accurate representationof the pH of the formation fluid at formation conditions.

Other methods of determining a pH of formation fluids includeintroducing a dye (e.g., phenol red, methylene blue, and/or cresol red)into the formation and correlating the pH of the formation fluid to thecolor of the dye. However, such dyes may not be formulated to determinethe pH of the formation fluid with a desired level of accuracy. Forexample, some dyes may only be sensitive within a narrow pH range, suchas a pH range of about 3.0 pH units. In addition, the dyes may bechemically unstable under formation conditions. Further, a continuous pHmeasurement may not be obtained unless the dye is continuously injectedinto the subterranean formation.

Other properties of the subterranean formation (e.g., salinity,wettability of formation surfaces, flow paths through the subterraneanformation, etc.) may be determined using one or more tracer compounds.For example, water tracers may be introduced into the subterraneanformation to estimate flow patterns between wells during enhanced oilrecovery processes, such as, for example, water flooding.

In addition to measuring at least one property within the formationthrough which the wellbore extends, tracers have been used in reservoirmonitoring. Reservoir monitoring refers to the gathering and analysis ofinformation from reservoirs during production. Such monitoring is usedto assess the productivity of producing formations or zones within theformations from which fluids are being produced. Monitoring of producedfluids is important in order to increase efficiency of a hydraulicfracturing operation. Reservoir monitoring is further used to determinewater saturation levels in the well.

In the past, produced fluids have been monitored by the use of tracersplaced in packs in strategic areas within the well. See, for instance,U.S. Pat. Nos. 3,991,827; 4,008,763; 5,892,147 and 7,560,690.

Tracers may include a fluorophore (i.e., a compound that can re-emitlight upon light excitation) and a presence of the tracer may bedetermined by optical spectroscopy (absorbance, fluorescence andphosphorescence). However, the fluorophore may include organic moleculesand rare-earth complexes that are toxic and/or radioactive and maycontaminate the subterranean formation (e.g., aquifers located in thesubterranean formation). Further, fluorophores may decompose at downholeconditions and may be subject to photobleaching (i.e., the photochemicalalteration of the fluorophore such that it becomes permanently unable tofluoresce) and photo blinking (i.e., fluorescence intermittency).

BRIEF SUMMARY

Embodiments disclosed herein include systems and methods for determiningat least one property of a subterranean formation. Additionalembodiments disclosed herein include methods of enhancing theproductivity of hydrocarbon containing fluids from a subterraneanformation penetrated by a well.

For example, in accordance with one embodiment, a system for determiningat least one property of at least one fluid in at least one subterraneanformation comprises a fluid delivery system configured and positioned todeliver a fluid into at least one of at least one subterranean formationand a wellbore extending through the at least one subterraneanformation, a radiation source within the wellbore, the radiation sourceconfigured to generate excitation radiation, carbon quantum dotsdisposed in the fluid, and a detector within the wellbore, the detectorconfigured to measure at least one optical property of the carbonquantum dots.

In additional embodiments, a system for determining at least oneproperty of at least one subterranean formation comprises at least onefiber optic cable within a wellbore extending through at least onesubterranean formation, the at least one fiber optic cable including atleast one optical fiber comprising carbon quantum dots, a radiationsource coupled to the at least one optical fiber, the radiation sourceconfigured to generate excitation radiation for transmission through theat least one optical fiber, and a detector coupled to the at least onefiber optic cable, the detector configured to measure at least oneoptical property of the carbon quantum dots.

In further embodiments, a method of forming carbon quantum dotscomprises providing an electrolyte comprising a carbon source and asource of ions to an electrochemical cell, introducing the electrolytebetween platinum electrodes of the electrochemical cell, and applyingelectrical current between the platinum electrodes to form carbonquantum dots including carbon from the carbon source.

In further embodiments, a method of determining at least one propertywithin at least one subterranean formation comprises introducing atleast one fiber optic cable into at least one of at least onesubterranean formation and a wellbore extending into the at least onesubterranean formation, transmitting excitation radiation through the atleast one fiber optic cable from a radiation source coupled to the atleast one fiber optic cable, exposing carbon quantum dots disposed in afluid in the wellbore or on the at least one fiber optic cable to theexcitation radiation, receiving, at an optical sensor coupled to the atleast one fiber optic cable, an emitted radiation from the carbonquantum dots responsive to exposure of the carbon quantum dots to theexcitation radiation, and measuring at least one of an emission spectrumand a fluorescence intensity of the emitted radiation at a detectorcoupled to the at least one fiber optic cable.

In additional embodiments, a method of fracturing multiple zones of asubterranean formation penetrated by a well comprises: (a) pumping intoeach zone of the formation to be fractured a fracturing fluid, whereinthe fracturing fluid pumped into each zone comprises a qualitativelydistinguishable tracer comprising carbon quantum dots which are eitherhydrocarbon soluble, water soluble or both hydrocarbon soluble and watersoluble; (b) enlarging or creating a fracture in the formation; (c)recovering fluid from at least one of the multiple zones; and (d)identifying the zone within the subterranean formation from which therecovered fluid was produced by identifying the carbon quantum dots inthe recovered fluid.

In other embodiments, a method of monitoring the production of fluidsproduced in multiple productive zones of a subterranean formationpenetrated by a well comprises: (a) pumping fracturing fluid into themultiple productive zones at a pressure sufficient to enlarge or createfractures in each of the multiple productive zones, wherein thefracturing fluid comprises optically active carbon quantum dots whichare either hydrocarbon soluble, water soluble or both hydrocarbonsoluble and water soluble and further wherein the fluorescent carbonquantum dots pumped into each of the multiple productive zones isqualitatively and/or quantitatively distinguishable; and (b) monitoringthe amount of fluids produced from at least one of the multipleproductive zones from the carbon quantum dots in the produced fluid.

In other embodiments, a method for enhancing the production ofhydrocarbons from a production well penetrating a hydrocarbon-bearingformation, wherein one or more injector wells are associated with theproduction well, comprises: (a) introducing into one or more of theinjector wells an aqueous fluid comprising fluorescent carbon quantumdots; (b) flowing at least a portion of the aqueous fluid comprising theoptically active carbon quantum dots from the one or more injector wellsinto the production well; and (c) recovering hydrocarbons from theproduction well.

In yet other embodiments, a method for determining water breakthrough ina production well associated with one or more injector wells, comprises:(a) injecting an aqueous fluid comprising optically active carbonquantum dots as tracer into an injector well; (b) flowing the aqueousfluid from the injector well into the production well; (c) producingfluid from the production well; and (d) determining water breakthroughin the production well by qualitatively determining the presence orquantitatively measuring the amount of the fluorescent carbon quantumdots in the produced fluid.

In still other embodiments, a method of increasing hydrocarbonproduction from a production well penetrating a hydrocarbon-bearingreservoir, wherein more than one injection well is associated with theproduction well, comprises: (a) injecting an aqueous fluid having awater-soluble tracer comprising carbon quantum dots into the more thanone injection well and maintaining pressure in the hydrocarbon-bearingreservoir above the bubble point of the hydrocarbons in the reservoir,wherein the aqueous fluid pumped into each of the injection wellscontains qualitatively distinguishable carbon quantum dots; (b)identifying from hydrocarbons recovered from the production well, uponwater breakthrough in the production well, the injection well into whichthe breakthrough water was injected by qualitatively determining thepresence of the carbon quantum dots in the recovered hydrocarbons; and(c) shutting off the injector well identified in step (b).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic illustrating a system including awellbore within a subterranean formation, in accordance with embodimentsof the disclosure;

FIG. 2A is a simplified cross-sectional view illustrating a fiber opticcable, in accordance with embodiments of the disclosure;

FIG. 2B is a simplified cross-sectional view of the fiber optic cabletaken along section line B-B of FIG. 2A;

FIG. 2C is a simplified cross-sectional view of another fiber opticcable, in accordance with embodiments of the disclosure;

FIG. 2D is a simplified cross-sectional view illustrating a fiber opticcable, in accordance with other embodiments of the disclosure;

FIG. 2E is a simplified schematic illustrating a measuring systemincluding a fluid delivery system, in accordance with yet otherembodiments of the disclosure;

FIG. 3A is a graph illustrating an absorption spectrum, an excitationspectrum, and an emission spectrum of carbon quantum dots, in accordancewith embodiments of the disclosure;

FIG. 3B is a graph illustrating a change in intensity and a change inwavelength as a function of pH for carbon quantum dots exposed to anexcitation radiation having a substantially monochromatic wavelength, inaccordance with embodiments of the disclosure; and

FIG. 4 is a simplified cross-sectional view of an electrochemical cellfor forming carbon quantum dots, in accordance with embodiments of thedisclosure.

DETAILED DESCRIPTION

The following description provides specific details, such as materialtypes, compositions, and processing conditions in order to provide athorough description of embodiments of the disclosure. However, a personof ordinary skill in the art will understand that the embodiments of thedisclosure may be practiced without employing these specific details.Indeed, the embodiments of the disclosure may be practiced inconjunction with conventional techniques employed in the industry. Inaddition, the description provided below does not form a completeprocess flow for measuring properties within a subterranean formation orfor forming carbon quantum dots. Only those process acts and structuresnecessary to understand the embodiments of the disclosure are describedin detail below.

As used herein, the term “optical property” means and includes anyqualitative or quantitative property relating to optically active carbonquantum dots (CQDs) which may be determined by optical spectroscopy(such as absorbance, fluorescence and phosphorescence). As non-limitingexamples, optical properties include a wavelength at which a materialexhibits a peak absorption intensity, a wavelength at which a materialexhibits a peak fluorescence intensity (e.g., a color of light emittedduring fluorescence, such as when the fluorescence is in the visiblespectrum), an excitation spectrum, an emission spectrum, an intensity ofabsorbed electromagnetic radiation, and an intensity of emittedelectromagnetic radiation. The electromagnetic radiation may be anywherewithin the electromagnetic spectrum, including, for example, the UVspectrum, the visible spectrum, and the infrared (IR) spectrum.

According to embodiments disclosed herein, a method of forming carbonquantum dots (CQDs) includes providing an electrochemical cell includingan electrolyte comprising a carbon source, water, and at least anothermaterial. A current is applied across electrodes of the electrochemicalcell to form carbon quantum dots comprising carbon from the carbonsource. The carbon source may include at least one of (i.e., one or moreof) nitrogen, boron, silicon, and phosphorus to form at least one ofnitrogen-doped, boron-doped, silicon-doped, and phosphorus-doped carbonquantum dots, respectively. The carbon quantum dots may be watersoluble, exhibit unique optical properties depending on a size andchemical composition (e.g., doping) of the carbon quantum dots, may bestable at wide pH ranges and temperatures (e.g., up to about 400° C.),and may be resistant to photobleaching and photo blinking. Surfaces ofthe carbon quantum dots may be functionalized to form exposedhydrophilic surfaces, exposed hydrophobic surfaces, or exposedamphiphilic surfaces on the carbon quantum dots.

The optical properties of carbon quantum dots may be used to determineat least one property of at least one subterranean formation penetratedby a well (e.g., a pH of the formation fluid, a wettability of formationsurfaces, a production zone within the at least one subterraneanformation, an injection well contributing to the flow of breakthroughwater, etc.). For example, the carbon quantum dots may exhibit anoptical property that is related to a pH to which the carbon quantumdots are exposed.

Carbon quantum dots thus may be introduced into the subterraneanformation penetrated by a well and exposed to excitation radiation(e.g., an excitation wavelength), such as fluorescence.

The well may be an oil well, gas well, water well or a geothermal well.

The radiation source may be located within the wellbore and may beconfigured to provide the excitation radiation to the carbon quantumdots disposed within the fluid and be configured to measure at least oneoptical property of the carbon quantum dots. For example, a radiationsource (e.g., a light source) may be coupled to a fiber optic cable,which may transmit the excitation radiation to the carbon quantum dots.The carbon quantum dots may be disposed within at least one opticalfiber of the fiber optic cable or may be coated onto at least a portionof the at least one optical fiber. Responsive to exposure to theexcitation radiation, the carbon quantum dots may fluoresce (e.g.,re-emit radiation at a different wavelength than the excitationwavelength).

The emitted radiation may be transmitted through the at least oneoptical fiber to a detector that may be configured to measure at leastone optical property of the carbon quantum dots.

The detection source may further be located above the wellbore. Forinstance, the source may be on the fly or at an external locationdistant from the wellbore.

In some embodiments, the carbon quantum dots are disposed in a fluidwithin the wellbore. In one embodiment, the carbon quantum dots may beintroduced or pumped into a well as neutrally buoyant particles in thecarrier fluid.

A fluid delivery system may be configured to provide (e.g., deliver) thecarbon quantum dots to the wellbore. The carbon quantum dots arecompatible with fluids naturally present in the reservoir and within therock itself. In addition, the carbon quantum dots are compatible withthe fluids injected into the reservoir as part of the formationtreatment. Further, the carbon quantum dots must be susceptible to beingreadily detected qualitatively and analyzed quantitatively in thepresence of the materials in the formation fluids.

The carbon quantum dots may be used to identify fluids produced from thewell. Since the carbon quantum dots may be detected in recoveredproduced fluids, the methods described herein thus do not requiredownhole equipment for detection. Typically, fluids transported out ofthe well are evaluated and the carbon quantum dots are identified on thefly or at a location distant from the wellbore.

In addition, the carbon quantum dots may be used as tracers to monitorfluid flow through the subterranean formation. For example, carbonquantum dots exhibiting different optical properties may be introducedinto different zones (e.g., producing zones, aquifer zones, etc.) of thesubterranean formation. A produced fluid exhibiting an optical property,such as fluorescence, corresponding to a property of carbon quantum dotsintroduced into a zone of the subterranean zone may be an indicationthat the produced fluid originated from the zone in which the carbonquantum dots were introduced.

The carbon quantum dots may also be used to sweep a production well inan enhanced oil recovery (EOR) operation, such as flooding. Carbonquantum dots may be introduced into injection fluid and the injectionfluid introduced into the formation. The injection fluid may beintroduced by being pumped into one or more injection wells. Typically,the carbon quantum dots are soluble in the injection fluid.

Injection fluids transporting carbon quantum dots into the formation aretypically aqueous based, such as a brine, like a saturated potassiumchloride or sodium chloride brine, salt water such as seawater, freshwater, a liquid hydrocarbon, a surfactant or a gas such as nitrogen orcarbon dioxide. The carbon quantum dots may further be injected into theformation in liquefied gas, such as liquefied natural gas or liquefiedpetroleum gas as well as in foams, such as carbon dioxide, nitrogen andcarbon dioxide/nitrogen. The injection fluid is preferably aqueous,steam or gas (water flooding, steam flooding or gas flooding).

The detection of the carbon quantum dots in fluids produced from theproduction well is indicative that the sweep, i.e., removal of the oilfrom pore spaces within the formation, has been completed.

Injection fluid pumped into the production well at different locationsmay contain qualitatively and/or quantitatively different carbon quantumdots. For instance, different carbon quantum dots, distinguishable fromeach other, may be introduced in an aqueous fluid into differentinjection wells. Fluids produced from the well may be analyzed todetermine if water breakthrough has occurred in the production well. Byusing different carbon quantum dots in different fluids, the injectionwell from which the water in the breakthrough water was pumped may bedetermined by optical spectroscopy. The injection well, into which thewater in the breakthrough water has been determined to have beeninitially introduced, can be shut off. Thus, the carbon quantum dots canbe used to optimize enhancement of hydrocarbons during secondaryrecovery operations by shutting down the injection well feeding into theformation into which sweep efficiency has been maximized. Thus, the flowof water from the injection well into that portion of the formationhaving been completely swept may be terminated.

Generally, fluids pumped into the production well do not requireexcessive amounts of the carbon quantum dots. Typically, the minimumamount of carbon quantum dots in the fluid introduced into theformation, the production well or injection well is that amountsufficient to permit detection within a produced fluid. Typically, theamount of carbon quantum dots present in the introduced fluid is betweenfrom about 1 ppm to about 500,000 ppm.

In some embodiments, a mixture of hydrophilic and hydrophobic carbonquantum dots exhibiting different optical properties may be introducedinto the subterranean formation, into the production well or into one ormore injection wells.

A ratio of hydrophilic carbon quantum dots to hydrophobic carbon quantumdots in a produced fluid may be determined by an optical property of theproduced fluid. The ratio may be employed as an indication of awettability of surfaces of the subterranean formation (e.g., a ratio ofwater wet surfaces to oil wet surfaces in the subterranean formation).It may also be indicative of the productivity of particular zones withinthe formation.

FIG. 1 is a simplified schematic illustration of a wellbore system 100extending through one or more subterranean formations. The subterraneanformations may include a plurality of zones, including a first zone 101proximate a surface of the earth, an aquifer zone 102 below the firstzone, a second zone 103 below the aquifer zone 102, a third zone 104below the second zone 103, a fourth zone 105 below the third zone 104,and a fifth zone 106 horizontally adjacent to the fourth zone 105. Thesubterranean formation may include one or more additional zones, such asa sixth zone 107 horizontally adjacent to the fifth zone 106. At leastsome of the zones may be hydrocarbon-bearing zones. For example, thesecond zone 103 and the fifth zone 106 may be hydrocarbon-bearing zonesand may include fractures 116 through which hydrocarbons to be producedmay travel during production. The other zones (e.g., the third zone 104,the fourth zone 105, and the sixth zone 107) may also containhydrocarbons. Each of the zones may be isolated from other zones by atleast one packer 108.

A wellbore 110 may extend through each of the different zones of thesubterranean formation. Cement 112 may line the wellbore 110 at leastthrough the first zone 101, the aquifer zone 102, and at least a portionof the second zone 103. A liner string 113 may line at least a portionof the wellbore 110. A production string 114 may extend through thesubterranean formation and to a portion of the formation bearinghydrocarbons to be produced.

During formation and operation of the wellbore 110 (e.g., duringdrilling, completion, stimulation, production, etc.), it may bedesirable to measure or estimate properties of fluids (e.g., drillingfluids, stimulation fluids, completion fluids, formation fluids,injection fluids, produced fluids etc.) located within the wellbore 110and/or subterranean formation through which the wellbore 110 extends.Further, it may desirable to determine the productivity of the zoneswithin the formation they have been subjected to fracturing in amulti-zone fracturing operation, water breakthrough in fluids producedfrom the formation, sweep efficiency, etc. Such properties may bemeasured in real time.

Thus, as will be described in more detail below, at least one opticalproperty, such as at least one of (i.e., one or more of) an absorptionspectrum, an absorption intensity, a peak absorption wavelength, anemission spectrum, a peak emission wavelength, and a fluorescenceintensity of carbon quantum dots, may be related to the determination ofat least one property of at least one subterranean formation asreferenced herein. In addition, the at least one optical property mayidentify one or more injection wells from which water has beenintroduced into a production well, etc.

The carbon quantum dots may be formulated to exhibit unique opticalproperties associated with the size and the molecular composition of thecarbon quantum dots. Carbon quantum dots which are qualitativelydistinguishable by detection means and/or quantitatively distinguishableby detection means may be used. For instance, in the production ofproduced fluids from a formation, a first group of carbon quantum dotsand at least a second group of carbon quantum dots may be used; thefirst group of carbon quantum dots formulated to exhibit a differentdetection property than the at least a second group of carbon quantumdots. At least one group of carbon quantum dots may be soluble in waterand at least one other group of carbon quantum dots may be soluble inhydrocarbon. Alternatively, both groups may be soluble in hydrocarbon orwater.

The carbon quantum dots may be formulated to fluoresce at wavelengthscorresponding to a color of the visible spectrum (e.g., violet, blue,cyan, green, yellow, orange, and red). The color of fluorescence maydepend at least in part upon at least one of a size and a chemicalcomposition of the carbon quantum dots. In some embodiments, the carbonquantum dots may be formulated to exhibit upconversion properties. Forexample, in some embodiments, the carbon quantum dots may be formulatedto emit radiation at a shorter wavelength (and a corresponding higherenergy) than radiation absorbed by the carbon quantum dots.

Accordingly, carbon quantum dots may be introduced into the subterraneanformation at a zone where it is desired to determine the pH of a fluidwithin the wellbore (e.g., formation fluid). In some embodiments, thecarbon quantum dots comprise a part of at least one optical fiber (e.g.,the carbon quantum dots may comprise a coating on an optical fiber orthe carbon quantum dots may be disposed within the optical fiber). Theoptical fiber including the carbon quantum dots may be exposed to fluidin communication with the subterranean formation.

In other embodiments, the carbon quantum dots are introduced into thesubterranean formation with a fluid delivery system configured todeliver a fluid having the carbon quantum dots suspended therein to thesubterranean formation.

In other embodiments, the carbon quantum dots are introduced into one ormore injection wells in a fluid delivery system configured to deliver afluid into the injection well and to maintain pressure within thewellbore above the bubble point of fluids being extracted from theformation.

The pH of a fluid within the wellbore 110 may be determined by exposingthe carbon quantum dots disposed within the wellbore 110 to anexcitation radiation and measuring at least one of (i.e., one or moreof) the absorption spectrum, the absorption intensity, the peakabsorption wavelength (i.e., the peak of the absorption spectrum), theemission spectrum, the peak emission wavelength (i.e., the peak of theemission spectrum), and the fluorescence intensity of the carbon quantumdots responsive to exposure to the excitation radiation. The excitationradiation may be at a substantially monochromatic wavelength or may beat a plurality of wavelengths (i.e., polychromatic wavelengths).

With continued reference to FIG. 1, the wellbore system 100 may includea fiber optic cable 120 extending from a surface location of thesubterranean formation to locations adjacent to one or more zones withinthe subterranean formation. The fiber optic cable 120 may extend alongan interior of the production string 114, similar to a wireline, as isknown to those of ordinary skill in the art, and may be run into theproduction string 114 as desired, or permanently deployed within theproduction string 114. Although the fiber optic cable 120 is illustratedas extending along an interior of the production string 114, the fiberoptic cable 120 may be located at any suitable location within thewellbore system 100 relative to the production string 114. For example,the fiber optic cable 120 may be run along an exterior of the productionstring 114, or comprise part of a self-contained sensor packageconfigured for wireless communication, as noted below.

The fiber optic cable 120 may be coupled to a radiation source 122 andto a detector 124. In some embodiments, the radiation source 122 and thedetector 124 may be located at the surface above the subterraneanformation, such as on or adjacent to the rig floor. As will be describedherein, in other embodiments, one or more of the radiation source 122and the detector 124 may be located within the wellbore 110. Theradiation source 122 may be configured to emit electromagnetic radiationat one or more wavelengths (i.e., the excitation radiation), which maybe transmitted through the fiber optic cable 120 to one or morelocations within the subterranean formation. In some embodiments, theradiation source 122 comprises a laser configured to transmit theexcitation radiation at a substantially monochromatic (e.g., asubstantially fixed and uniform) wavelength. The substantiallymonochromatic wavelength may be any wavelength in the electromagneticspectrum. In some embodiments, the substantially monochromaticwavelength may be within the ultraviolet spectrum, such as, for example,between about 100 nm and about 400 nm. In other embodiments, theradiation source 122 includes a broadband radiation source configured toprovide the excitation radiation at more than one wavelength (e.g.,polychromatic wavelengths). By way of non-limiting example, theradiation source 122 may include a light-emitting diode (LED) (e.g., acollimated LED, an uncollimated LED), a xenon lamp, a mercury lamp, orother suitable electromagnetic radiation source. In some embodiments,the excitation radiation is transmitted in pulses.

The fiber optic cable 120 may include one or more optical sensors 126configured to detect one or more fluorescence properties of the carbonquantum dots in the wellbore system 100. FIG. 2A is a simplifiedschematic representation of a fiber optic cable 120 including an opticalsensor 126. The fiber optic cable 120 may include at least one opticalfiber 128 within a sheath 132 configured to transmit the excitationradiation to the carbon quantum dots within the wellbore 110 and atleast one optical fiber 130 within the sheath 132 configured to receivethe radiation emitted from the carbon quantum dots. Each of the opticalfibers 128 may be coupled to the radiation source 122 (FIG. 1) and eachof the optical fibers 130 may be coupled to the detector 124 (FIG. 1).The optical sensor 126 may include at least one exposed portion of theoptical fiber 128 and at least one exposed portion of the optical fiber130.

Each of the optical fibers 128 may be configured to receive theexcitation radiation independently of other optical fibers 128 and atdiffering wavelengths, intensities and, if applicable, pulse rates,radiation pulses from different optical fibers 128 being sentsimultaneously or at offset time intervals. In other embodiments, eachof the optical fibers 128 may be configured to receive excitationradiation of substantially the same wavelength, intensity and, ifapplicable, pulse rates and intervals as the other optical fibers 128.In yet other embodiments, the radiation source 122 may be configured toprovide the excitation radiation at a substantially monochromaticwavelength and intensity to one of the optical fibers 128 and excitationradiation of another substantially monochromatic wavelength andintensity to another of the optical fibers 128.

A distal end of the optical fiber 128 may include what is known in theart as a “mirror finished” or a “polished” end 134. The mirror finishedends 134 of the optical fibers 128 may be angled with respect to alongitudinal axis of the optical fiber 128 and may be configured toreduce undesired reflection and/or scattering of the excitationradiation. For example, the mirror finished end 134 may be configured toreduce attenuation of the excitation radiation to be received throughthe optical fibers 130. The mirror finished ends 134 may be configuredto substantially reflect light emitted by the carbon quantum dots to thedetector 124.

At least a portion of at least one optical fiber 128 may include carbonquantum dots 129 disposed therein. The carbon quantum dots 129 may bedisposed within one or more optical fibers. FIG. 2B is a simplifiedcross-sectional view of the fiber optic cable 120 of FIG. 2A. The carbonquantum dots 129 may be disposed within and integral with the opticalfibers 128. By way of non-limiting example, carbon quantum dots 129 maybe dispersed in a composition (e.g., mixed in a molten solution) fromwhich the optical fibers 128 are formed (e.g., extruded, drawn, cast,etc.). It is contemplated that, in some embodiments, the optical fibers128 may include materials formulated to enhance optical properties ofthe optical fibers 128, such as, for example, titanium dioxide.

Portions of the optical fibers 128 may be exposed to a wellbore fluid140 (e.g., drilling fluids, stimulation fluids, completion fluids,formation fluids, etc.). For example, at least a distal end of theoptical fibers 128 may be exposed to the wellbore fluid 140. Theportions of the optical fibers 128 that are exposed to the wellborefluid 140 may include the carbon quantum dots 129 disposed therein. Theexcitation radiation from the radiation source 122 may be transmitted tothe carbon quantum dots 129 of the optical fibers 128. Responsive toexposure to the excitation radiation, the carbon quantum dots 129 mayemit radiation exhibiting at least one fluorescence property related tothe pH of the formation fluid 140 surrounding the exposed portions ofthe optical fibers 128. The optical fiber 130 may receive the radiationemitted by the carbon quantum dots 129 and transmit the emittedradiation to the detector 124.

In other embodiments, the carbon quantum dots 129 may be disposed on atleast one surface of at least one optical fiber. For example, a surfaceof at least one optical fiber may have a coating of the carbon quantumdots. FIG. 2C is a simplified cross-sectional view of a fiber opticcable 120′ substantially similar to the fiber optic cable 120 of FIG.2B, except that the fiber optic cable 120′ includes optical fibers 128′having a coating 127 of carbon quantum dots thereon. In someembodiments, the coating 127 comprises a monolayer of carbon quantumdots. The coating 127 may substantially surround each of the opticalfibers 128′. The coating 127 may be a substantially continuous layeraround an entire circumference of each of the optical fibers 128′. Thecoating 127 may be in contact with the wellbore fluid 140, which mayaffect at least one fluorescence property of the carbon quantum dots ofthe coating 127.

The coating 127 may be located at, for example, the distal end of theoptical fiber 128′. The excitation radiation from the radiation source122 may be transmitted to the carbon quantum dots on the coating 127.Responsive to exposure to the excitation radiation, the carbon quantumdots may emit radiation exhibiting at least one fluorescence propertyrelated to the pH of the wellbore fluid 140 surrounding the coating 127.

The optical fibers 130 may be configured to receive the radiationemitted by the carbon quantum dots (e.g., radiation emitted from thecoating 127) and transmit the emitted radiation to the detector 124,which may be located at a surface location. Each of the optical fibers130 may be coupled to the detector 124.

Accordingly, in some embodiments, the carbon quantum dots may beintroduced into the subterranean formation with the fiber optic cable120, 120′. Radiation emitted by the carbon quantum dots on or within theoptical fibers 128, 128′ may be received by the optical fiber 130 andtransmitted to the detector 124. Thus, the carbon quantum dots may beconfigured to continuously measure the pH of the fluid in the wellbore110 without continuously introducing new carbon quantum dots into thesubterranean formation.

In other embodiments, the carbon quantum dots may not be coated on theoptical fibers 128, 128′, but may be disposed in the wellbore fluid 140.FIG. 2D illustrates an embodiment of another fiber optic cable 120″according to another embodiment of the disclosure. The fiber optic cable120″ may include an optical sensor 126′ comprising optical fibers 128″configured to transmit excitation radiation to carbon quantum dotsdisposed within the wellbore 110 and at least one optical fiber 130within the sheath 132 configured to receive the radiation emitted fromthe carbon quantum dots. The carbon quantum dots may be disposed in thewellbore fluid 140 proximate the optical fibers 128″, 130. Theconcentration of the carbon quantum dots in the wellbore fluid 140 maybe between about 50 parts per trillion (ppt) and about 10,000 parts permillion (ppm), such as between about 50 ppt and about 500 ppt, betweenabout 500 ppt and about 5,000 ppt (5 ppm), between about 5 ppm and about500 ppm, or between about 500 ppm and about 10,000 ppm.

Excitation radiation may be transmitted through the optical fibers 128″to the carbon quantum dots in the wellbore fluid 140. Responsive toexposure to the excitation radiation, the carbon quantum dots may emitradiation that may be received by the optical fibers 130. The opticalfiber 130 may transmit the emitted radiation to the detector 124. Thus,a pH of the fluid 140 proximate the optical fibers 128″, 130 may bedetermined by disposing the carbon quantum dots in the wellbore fluid140 and detecting at least one fluorescence property of the carbonquantum dots.

Accordingly, with reference again to FIG. 1, the radiation source 122may be configured to pulse the excitation radiation to the carbonquantum dots within the wellbore 110. Carbon quantum dots proximate oneor more of the optical sensors 126 may absorb the excitation radiation.Responsive to absorbing the excitation radiation, the carbon quantumdots may fluoresce at an emission wavelength (e.g., that may correspondto, for example, red light, yellow light, blue light, etc.). Duringfluorescence, the carbon quantum dots may re-emit radiation at awavelength (i.e., an emission wavelength) that is different from thewavelength of the excitation radiation (i.e., the excitationwavelength).

The detector 124 may be configured to continuously measure at least onefluorescence property (e.g., one or more of the absorption spectrum, thepeak absorption wavelength, the absorption intensity, the emissionspectrum, the peak emission wavelength, and the fluorescence intensity)of the carbon quantum dots. The measured fluorescence property may becorrelated to a pH of the formation fluid. Accordingly, the pH of theformation fluid may be measured in situ and in real time. The detector124 may include or be coupled to a processor configured to estimate thepH of the formation fluid based on one or more of the absorptionspectrum, the peak absorption wavelength, the absorption intensity, theemission spectrum, the peak emission wavelength, and the fluorescenceintensity of the carbon quantum dots. In some embodiments, the detectoris a spectrometer, such as a fluorescence spectrometer.

Although FIG. 2A through FIG. 2D illustrate optical fibers 128, 128′,128″ configured to transmit the excitation radiation to the carbonquantum dots and optical fibers 130 configured to transmit the emittedradiation to the detector 124, it is contemplated that in someembodiments, the fiber optic cable 120 may include a single opticalfiber. Excitation radiation may be transmitted through the optical fiberin pulses, such as every millisecond, every 10 milliseconds, or every100 milliseconds. The fluorescence emitted by the carbon quantum dotsmay be transmitted back through the single optical fiber betweenexcitation pulses and received by the detector 124. In other words, theexcitation pulses may be separated in time such that the carbon quantumdots may fluoresce and the emitted fluorescent radiation may be measuredat the detector 124 in between consecutive pulses of excitationradiation. Although FIG. 1 illustrates only one fiber optic cable 120extending into the wellbore 110, the wellbore system 100 may include aplurality of fiber optic cables 120 extending into the wellbore 110. Forexample, in some embodiments, at least one fiber optic cable 120 may beconfigured to transmit the excitation radiation to the carbon quantumdots and at least one fiber optic cable 120 may be configured to receiveand transmit the emitted radiation from the carbon quantum dots to thedetector 124.

Although the radiation source 122 and the detector 124 are illustratedas being located at a surface location of the subterranean formation, atleast one of the radiation source 122 and the detector 124 may belocated within the wellbore 110. FIG. 2E is a simplified schematicillustrating a measuring system 150 according to another embodiment ofthe disclosure. The measuring system 150 includes a fluid deliverysystem 152 configured and positioned to deliver a carbon quantumdot-containing fluid 154 into the wellbore 110, such as into theproduction string 114. The radiation source 122 and the detector 124 maybe located at a location downstream of the fluid delivery system 152.The wellbore fluid 140 may flow in the production string 114 in thedirection indicated by arrow 156. The wellbore fluid 140 may carry thecarbon quantum dot-containing fluid 154 to a location proximate theradiation source 122 and the detector 124. In some embodiments, thefluid delivery system 152 is located proximate the radiation source 122and the detector 124, such as, for example, within about one meter ofthe radiation source 122 and the detector 124. The carbon quantumdot-containing fluid 154 may substantially mix with the wellbore fluid140 prior to being exposed to excitation radiation 160 from theradiation source 122.

As the carbon quantum dots in the carbon quantum dot-containing fluid154 are exposed to the excitation radiation 160 from the radiationsource 122, the carbon quantum dots may fluoresce. Responsive toexposure to the excitation radiation 160, the carbon quantum dots mayemit radiation that may be received by the detector 124, which, in someembodiments, may be located directly across from the radiation source122. In other embodiments, the detector 124 may be located adjacent theradiation source 122, such that the carbon quantum dots pass thedetector 124 directly after exposure to the excitation radiation 160.Accordingly, a pH of the wellbore fluid 140 may be determined bydisposing the carbon quantum dots into the wellbore fluid 140 (e.g., viathe carbon-quantum dot-containing fluid 154) and detecting at least onefluorescence property of the carbon quantum dots.

The detector 124 may be configured to transmit information about thedetected fluorescence properties to the surface, such as by, forexample, a wire 158 coupled to the detector 124 and configured totransmit the data to the surface, wireless communications, mud pulsetelemetry, or other method suitable to transmit the data from thedetector 124 located within the wellbore 110 to the surface of thesubterranean formation.

FIG. 3A illustrates an example of an absorption spectrum 202, anexcitation spectrum 204, and an emission spectrum 206 of fluorescence ofcarbon quantum dots in a solution. The absorption spectrum 202 (y-axisof the absorption spectrum 202 illustrated on the left side of FIG. 3A)graphs the absorption intensity of the carbon quantum dots as a functionof an excitation wavelength to which the carbon quantum dots areexposed. The peak absorption intensity occurs at a wavelength ofapproximately 275 nm. The excitation spectrum 204 (y-axis of theexcitation spectrum 204 illustrated on the right side of FIG. 3A) graphsa radiation intensity of the excitation radiation as a function of thewavelength. The peak excitation intensity occurs at an excitationwavelength of approximately 355 nm. Although the excitation spectrum 204is illustrated as shifted from the absorption spectrum 202, in someembodiments, the excitation spectrum 204 may be more closely alignedwith the absorption spectrum 202. The emission spectrum 206 (y-axis ofthe emission spectrum 206 illustrated on the right side of FIG. 3B)graphs an intensity of the emitted fluorescence radiation emitted by thecarbon quantum dots as a function of wavelength. The emission spectrum206 illustrates that the peak fluorescence intensity occurs at anemission wavelength of approximately 480 nm (i.e., an emission ofblue-colored light). The intensity of the peak emission wavelength ofthe emission spectrum 206 may increase or decrease, depending upon thepH of the solution in which the carbon quantum dots are disposed. Thus,the carbon quantum dots are exposed to excitation radiation at asubstantially monochromatic wavelength and the wavelength of the peakemitted radiation (i.e., the wavelength of the peak fluorescenceintensity) is shifted from the peak wavelength of the excitationradiation.

As described above, the absorption spectrum and the emission spectrumemitted by the carbon quantum dots may depend on a pH of the solution inwhich the carbon quantum dots are disposed. Thus, for the sameexcitation wavelength, a change in the pH of the formation fluid inwhich the carbon quantum dots are disposed may correspond to a change inone or more of the absorption spectrum, the corresponding peakabsorption wavelength of excitation radiation absorbed by the carbonquantum dot, the emission spectrum, and the corresponding peak emissionwavelength emitted by the carbon quantum dots.

FIG. 3B illustrates a change in intensity (e.g., one or more of anabsorption intensity and an emission intensity) and a change inwavelength (e.g., a change in one or more of an excitation wavelengthand an emission wavelength) as a function of pH for carbon quantum dotsexposed to an excitation radiation at a substantially monochromaticwavelength. As illustrated in FIG. 3B, at a substantially monochromaticwavelength, the intensity of the carbon quantum dots (e.g., at thewavelength at which the carbon quantum dots exhibit the mostfluorescence) may depend upon a pH of the solution in which the carbonquantum dots are disposed. Similarly, a wavelength at which carbonquantum dots absorb excitation radiation and an emission wavelength ofthe carbon quantum dots may change based on the pH to which the carbonquantum dots are exposed. Accordingly, in some embodiments, for asubstantially monochromatic excitation wavelength, a pH of a formationfluid may be estimated based at least in part on one or morefluorescence or absorption properties of the carbon quantum dots, suchas, for example, the absorption intensity (e.g., a change in theabsorption intensity), the fluorescence intensity (e.g., a change in thefluorescence intensity), a change in the absorption wavelength (e.g., achange in the wavelength at which a highest intensity of excitationradiation is absorbed), a change in the emission radiation wavelength(e.g., a change in the wavelength at which a highest fluorescenceintensity occurs), and combinations thereof. For example, at asubstantially monochromatic wavelength, the emission radiationwavelength of the carbon quantum dots may shift depending on the pH ofthe solution in which the carbon quantum dots are disposed.

Hydrocarbons within the subterranean formation may include materialsthat fluoresce responsive to exposure to the excitation radiation.Fluorescence of such materials may undesirably increase noise in atleast one of the fluorescence properties measured by the detector 124.However, such materials may have a shorter fluorescence lifetime than afluorescence lifetime of the carbon quantum dots. In some embodiments,the detector 124 may be configured to measure the at least onefluorescence property of the carbon quantum dots after a time delay,such as in time-resolved fluorometric detection. Measuring thefluorescence of the carbon quantum dots after a time delay may reducebackground noise caused by fluorescence of the materials in thehydrocarbons and increase the signal-to-noise ratio of the detector 124.The time delay may be between about 1 picosecond (ps) and about 100microseconds (μs), such as between about 1 picosecond and about 1nanosecond, between about 1 nanosecond and about 100 nanoseconds,between about 100 nanoseconds and about 1 microsecond, between about 1microsecond and about 10 microseconds, or between about 10 microsecondsand about 100 microseconds.

Accordingly, a fluid introduced into the subterranean formation mayinclude the carbon quantum dots, or the carbon quantum dots may compriseat least a portion of a fiber optic cable 120, 120′, such as within theat least one optical fiber 128 or as a coating on the at least oneoptical fiber 128′. The carbon quantum dots may be exposed to theexcitation radiation. As described above, responsive to exposure to theexcitation radiation, the carbon quantum dots may exhibit a fluorescenceproperty that is, at least partially, dependent upon the pH of the fluidsurrounding the carbon quantum dots. The emitted radiation may betransmitted from the carbon quantum dots to the detector 124, where atleast one fluorescence property of the carbon quantum dots may bemeasured. The pH of fluid in which the carbon quantum dots are disposedmay be determined based on a fluorescence property of the carbon quantumdots. The carbon quantum dots, either within the wellbore fluid 140 orwithin the at least one optical fiber 128, 128′, may be substantiallychemically inert (e.g., may not be subject to photobleaching) and mayremain within the fluid or within the at least one optical fiber 128,128′ when exposed to formation conditions.

FIG. 4 illustrates a simplified cross-sectional view of a configurationthat may be used in a method of forming the carbon quantum dotsdescribed herein. The method includes providing an electrolyte 304 andelectrodes 306 in a container 302 to form an electrochemical cell 300.Electrical current may be applied to the electrochemical cell 300 toform carbon quantum dots from a carbon source located in the electrolyte304.

The container 302 may be any vessel or container suitable for holdingthe electrolyte 304 before, during, or after the electrochemical processof the disclosure, as described in further detail below. By way ofnon-limiting example, the container 302 may comprise a glass beakerconfigured to receive and hold the electrolyte 304 and the electrodes306.

The electrodes 306 may include at least one anode and at least onecathode. In some embodiments, each of the electrodes 306 comprisesplatinum. The electrodes 306 may be coupled to a power supply configuredto provide an electrical current to the electrochemical cell 300. Acurrent may be applied to the electrochemical cell 300 for a sufficientperiod of time to form carbon quantum dots from the electrolyte 304. Byway of non-limiting example, the applied current density may be within arange extending from about 100 milliamperes per square centimeter(mA/cm²) to about 1,100 mA/cm² (e.g., from about 100 mA/cm² to about 500mA/cm², from about 500 mA/cm² to about 1,000 mA/cm², or from about 1,000mA/cm² to about 1,100 mA/cm²). In some embodiments, the applied currentdensity is approximately 1,100 mA/cm². A voltage may be applied betweenthe electrodes 306 during the electrochemical reaction process. In someembodiments, a voltage of approximately 10 volts may be applied betweenthe electrodes 306. Accordingly, the carbon quantum dots may be formedwithout using a carbon-containing electrode, such as a graphiteelectrode. Even when using an electrode that includes carbon, such as agraphite electrode, the carbon in the resulting carbon quantum dots maynot include any significant amount of carbon that originated from theelectrode.

Although FIG. 4 illustrates two electrodes 306, the electrochemical cell300 may include any number of electrodes 306 (e.g., three, four, five,etc.).

After a suitable period of time, carbon quantum dots may form in theelectrochemical cell 300. The electrolyte 304 may be evaporated and anysolids may be collected. The solids may include amorphous carbon quantumdots. Accordingly, carbon quantum dots exhibiting different fluorescenceproperties (e.g., peak emission wavelengths) may be formed in theelectrochemical cell 300.

The electrolyte 304 may include at least one carbon source formulatedfor providing carbon for forming the carbon quantum dots during theelectrochemical process. The electrolyte 304 may further include asource of ions, such as an acid, a base, or a buffer. In someembodiments, the source of ions includes a hydroxide, such as, forexample, sodium hydroxide (NaOH), potassium hydroxide (KOH), cesiumhydroxide (CsOH), magnesium hydroxide (Mg(OH)₂), calcium hydroxide(Ca(OH)₂), and barium hydroxide (Ba(OH)₂). In some embodiments, the atleast another material has a concentration of about 1 molar and may beformulated such that the electrolyte 304 has a pH between about 13 andabout 14.

The carbon source may constitute between about 1 volume percent andabout 100 volume percent of the electrolyte 304, such as between about 1volume percent and about 10 volume percent, between about 10 volumepercent and about 25 volume percent, between about 25 volume percent andabout 50 volume percent, and between about 50 volume percent and about100 volume percent. The carbon source may be dispersed in water. In someembodiments, a ratio of the carbon source to water is approximately oneto two (1:2).

The carbon source may include any water-soluble carbon-containingmaterial. In some embodiments, the carbon source is an alcohol, such asmethanol, ethanol, propanol, butanol, combinations thereof, etc. In someembodiments, the carbon source is ethanol. Carbon quantum dots formedfrom such carbon sources may comprise carbon, hydrogen, and oxygen(e.g., may be undoped).

The carbon-quantum dots may be formulated to include at least one ofnitrogen, boron, silicon, and phosphorus. The electrolyte 304 may beformulated to include at least one of a nitrogen source, a boron source,a silicon source, and a phosphorus source. The nitrogen source, theboron source, the silicon source, and the phosphorus source may alsoinclude carbon. Suitable nitrogen-containing carbon sources may includeamino alcohols, such as, for example, ethanolamine (C₂H₇NO),diethanolamine (C₄H₁₁NO₂), and triethanolamine (C₆H₁₅NO). Thenitrogen-containing carbon source may include a 2-aminoalcohol, such as,for example, 2-amino-1-propanol (alaninol) (C₃H₉NO),2-amino-1,3-propanediol (serinol) (C₃H₉NO₂), L-tryptophanol (C₁₁H₁₄N₂O),a 1-amino-2-propanol (C₃H₉NO), and a propanolamine, such as metoprolol(C₁₅H₂₅NO₃), nadolol (C₁₇H₂₇NO₄), and phenylpropanolamine (C₉H₁₃NO), orany other water-soluble carbon source including nitrogen.

Suitable boron-containing carbon sources may include water-solubleorganoboranes, such as, for example, a trialkylborane ((R₁R₂R₃B),wherein R₁, R₂, and R₃ are alkyl groups. Suitable trialkylboranes mayinclude, for example, trimethylborane ((CH₃)₃B), triethylborane((C₂H₅)₃B), and tripropylborane ((C₃H₇)₃B). Other boron-containingsources may include diborane (H₆B₂), a carborane, decaborane (B₁₀H₁₄), aboronic acid, such as, for example, phenylboronic acid (C₆H₇BO₂),methylboroinic acid (CH₃B(OH)₂), and propenylboronic acid (C₃H₅B(OH)₂).Other boron-containing sources may include a boratobenzene (aborabenzene), such as, for example, 1-boratanaphthalene,9-borataanthracene, boracyclooctantetraene, and 2,2′-diboratabiphenyl.

In some embodiments, the electrolyte 304 includes a compound including anitrogen source and a boron source. For example, the electrolyte 304 mayinclude a borane-amine complex, such as borane trimethylamine((CH₃)₃NBH₃) and borane tert-butylamine complex ((CH₃)₃CNH₂BH₃).

Suitable silicon-containing carbon sources may includehydroxyalkylsilanes, (e.g., hydroxymethyltrimethylsilane(HOCH₂Si(CH₃)₃), hydroxyethoxysilatrane (C₈H₁₇NO₅Si)), and otherwater-soluble organosilicon compounds.

Suitable phosphorus-containing compounds may include phosphate esterssuch as, for example, a phosphatidylcholine, triphenylphosphate(OP(OC₆H₅)₃), cyclophosphamide (C₇H₁₅C₁₂N₂O₂P), and parathion(C₁₀R₄NO₅PS), phosphonic acids and their esters, such as, for example,glyphosate (C₃H₈NO₅P), phosphoranes, such as, for example,pentaphenylphosphorane (P(C₆H₅)₅), and organophosphorus compounds, suchas, for example, triphenylphosphine (P(C₆H₅)₃), phosphites,phosphonites, and phosphinites.

Accordingly, the carbon quantum dots may be doped with at least one ofnitrogen, boron, silicon, and phosphorus. The fluorescence properties ofthe carbon quantum dots may depend on the composition of the electrolyte304 (e.g., the carbon source) from which the carbon quantum dots areformed.

At least one of the nitrogen-containing carbon source, theboron-containing carbon source, the silicon-containing carbon source,and the phosphorus-containing carbon source may constitute between about0 volume percent and about 100 volume percent of the carbon source, suchas between about 1 volume percent and about 10 volume percent, betweenabout 10 volume percent and about 25 volume percent, between about 25volume percent and about 50 volume percent, or between about 50 volumepercent and about 100 volume percent of the carbon source.

The carbon quantum dots may further include C.dbd.C bonds and C—Ofunctional groups. The carbon quantum dots may be undoped,nitrogen-doped, boron-doped, silicon-doped, phosphorus-doped, andcombinations thereof. For example, at least some of the carbon quantumdots may include one of nitrogen, boron, silicon, and phosphorus and atleast some of the carbon quantum dots may include at least another ofnitrogen, boron, silicon, and phosphorus.

Any of the carbon quantum dots referenced herein may be suitable for themethods disclosed herein, especially those relating to the use of thecarbon quantum dots in hydraulic fracturing including multi-zonefracturing, enhanced oil recovery, flooding, etc.

Typically, the carbon quantum dots referenced herein may be generallyspherical in shape having diameters ranging from between about 1 nm toabout 10 nm. The carbon quantum dots may be separated into narrower sizeranges by suitable methods, which may include dialysis. For example, thecarbon quantum dots may be passed through at least one membrane having apore size corresponding to a desired size of the carbon quantum dots.The separated carbon quantum dots may have a diameter ranging frombetween about 1 nm and about 3 nm, between about 3 nm and about 5 nm, orbetween about 5 nm and about 10 nm. Carbon quantum dots having differentsizes may exhibit different optical properties.

The carbon quantum dots may be soluble in aqueous-based solutions. Thecarbon quantum dots may include exposed hydroxyl groups, exposedcarboxyl groups, exposed ether groups and combinations thereof. In someembodiments, exposed surfaces of the carbon quantum dots may befunctionalized with at least one of additional hydrophilic functionalgroups or hydrophobic functional groups. Non-limiting examples ofhydrophilic groups include, for example, a hydroxyl group, a carboxylgroup, an amine group, a thiol group, an ether group and a phosphategroup. Non-limiting examples of hydrophobic groups include, for example,an alkyl group, an alkenyl group, an alkynyl group, and an aryl group.

In some embodiments, a hydrophilic group or a hydrophobic group may beattached to the carbon quantum dots in a condensation reaction or ahydrolysis reaction, such as described in U.S. patent application Ser.No. 14/169,432, filed Jan. 31, 2014, and titled “METHODS OF USINGNANO-SURFACTANTS FOR ENHANCED HYDROCARBON RECOVERY,” or a reactionmechanism described in U.S. patent application Ser. No. 14/519,496,filed Oct. 21, 2014, and titled “METHODS OF RECOVERING HYDROCARBONSUSING SUSPENSIONS FOR HYDROCARBON RECOVERY,” the disclosure of each ofwhich applications is hereby incorporated herein in its entirety by thisreference. For example, a hydrophilic precursor or a hydrophobicprecursor may include a hydrolyzable group and may be attached to asurface of the carbon quantum dots by hydrolyzing the hydrolyzablegroup. In other embodiments, a hydrophilic or hydrophobic group may beattached to the carbon quantum dots by a condensation reaction betweenthe carbon quantum dots and one of a hydrophilic precursor and ahydrophobic precursor.

The carbon quantum dots may be stable at elevated temperatures (e.g., upto about 400° C.) and a wide range of pH (e.g., a pH between about 0 andabout 14.0). Emission spectra of the carbon quantum dots may bedependent upon the size and composition of the carbon quantum dots.

The carbon quantum dots may be formulated to interact with surfaces ofthe subterranean formation. For example, exposed surfaces of the carbonquantum dots may be functionalized with at least one functional group,such as with at least one hydrophilic group, at least one hydrophobic(e.g., oleophilic) group, and combinations thereof (e.g., to formamphiphilic surfaces). Hydrophilic groups on surfaces of the carbonquantum dots may interact with water wet surfaces of the subterraneanformation and hydrophobic groups may interact with oil wet surfaces ofthe subterranean formation.

In some embodiments, the hydrophilic carbon quantum dots may beformulated to exhibit a different optical property than the hydrophiliccarbon quantum dots. For example, the hydrophilic carbon quantum dotsmay have a different size than the hydrophobic carbon quantum dots. Inother embodiments, the hydrophilic carbon quantum dots are doped with atleast one of nitrogen, boron, silicon, phosphorus, etc., and thehydrophobic carbon quantum dots are undoped or doped with at leastanother of nitrogen, boron, silicon, and phosphorus.

A mixture of hydrophilic and hydrophobic carbon quantum dots may beintroduced into the subterranean formation by pumping the mixture ofcarbon quantum dots into the well penetrating the formation. A producedfluid may include at least one of the hydrophilic carbon quantum dotsand the hydrophobic carbon quantum dots.

The proportion of hydrophilic carbon quantum dots to hydrophobic carbonquantum dots may be determined by, for example, comparing thefluorescence intensity at the peak emission wavelength of thehydrophilic carbon quantum dots to the fluorescence intensity at thepeak emission wavelength of the hydrophobic carbon quantum dots.

A ratio of formation surfaces that are water wet relative to formationsurfaces that are oil wet may correspond to a proportion of hydrophiliccarbon quantum dots to hydrophobic carbon quantum dots in the producedfluid.

Information about the wettability of the formation surfaces may beparticularly useful where stimulation methods include expensive fluids,such as those including surfactants, micellar fluids, or polymers. Wherethe formation includes more water wet surfaces than oil wet surfaces, anaqueous-based stimulation fluid may be used during further stimulationprocedures. Where the formation includes more oil wet surfaces thanwater wet surfaces, a non-polar stimulation fluid may be used duringfurther stimulation procedures.

In some embodiments, the carbon quantum dots may be introduced into thesubterranean formation during stimulation processes. Stimulationprocesses such as, for example, hydraulic fracturing (i.e., “fracking”)may be used to enhance hydrocarbon recovery from a hydrocarbon-bearingsubterranean formation. In hydraulic fracturing operations, hydraulicfractures may be created or enlarged by injecting a fluid containingadditives and including a suspended proppant material (e.g., sand,ceramics, etc.) into a targeted subterranean formation under elevatedpressure conditions sufficient to cause the hydrocarbon-bearingformation material to fracture. The carbon quantum dots may be includedin the fracturing fluid.

In addition to determining a chemical or physical parameter of theformation fluid (such as pH), it may be desirable to determine alocation (e.g., a zone) from which produced fluids (e.g., hydrocarbons,water, etc.) originate. It is contemplated that carbon quantum dotsexhibiting different optical properties may be introduced into variouszones of the subterranean formation (as well as on an optical fiber). Insome embodiments, between about one and about twenty different types ofcarbon quantum dots, each exhibiting one or more different opticalproperties than the other types of carbon quantum dots, may beintroduced into one or more different zones of the subterraneanformation.

As another example, carbon quantum dots may be introduced proximate theaquifer zone. Produced fluids may be analyzed to determine if theproduced fluids include an optical property of the carbon quantum dotsintroduced into the aquifer zone. Identification of the correspondingoptical property may be an indication that the produced fluid includeswater from the aquifer zone.

In some embodiments, the carbon quantum dots may be used to identify asource of fluids produced from a production well. Carbon quantum dotsintroduced into each zone of the subterranean formation may exhibit adifferent optical property than carbon quantum dots introduced intoother zones of the subterranean formation. In particular, by way ofnon-limiting example, the carbon quantum dots may be dispersed inproduced fluids to indicate the source of the hydrocarbons. The opticalproperty, such as fluorescence, of the carbon quantum dots in thehydrocarbons may be an indication of the source of the hydrocarbons.

Thus, in some embodiments, carbon quantum dots exhibiting differentoptical properties may be introduced into multiple zones of thesubterranean formation. (The term “zone” as used herein may refer toseparate formations within a well or separate areas within a singleformation within the well.) The carbon quantum dots introduced in onezone may be different from the carbon quantum dots introduced intoanother zone being treated. The carbon quantum dots introduced intodifferent zones are preferably qualitatively (and preferably alsoquantitatively) distinguishable in order to identify the zone or areawithin the formation from which a produced fluid originates. As such,the carbon quantum dots introduced into each of the zones being treatedpreferably exhibit unique absorption and optical properties such thatthe properties of carbon quantum dots introduced into one zone is unableto mask the properties of carbon quantum dots introduced into anotherzone.

For instance, carbon quantum dots having a first chemical composition(e.g., undoped, nitrogen-doped, boron-doped, silicon-doped,phosphorus-doped, and combinations thereof) may be introduced into afirst zone and carbon quantum dots having a different composition may beintroduced into a second zone. Detection of an optical property in aproduced fluid corresponding to an optical property of carbon quantumdots disposed in a zone of the subterranean formation may be anindication that the produced fluid originated from the correspondingzone. Detection of optical properties in the produced fluid thatcorrespond to carbon quantum dots introduced into different zones may bean indication that the produced fluid comprises formation fluidoriginating from each of the corresponding zones.

Thus, for instance, a first fluid having fluorescent carbon quantum dotsmay be introduced into a first zone of a formation. A second fluidhaving qualitatively distinguishable carbon quantum dots from the fluidintroduced into the first zone) may be introduced into a second zone ofa formation. A proportion of formation fluid originating from each zonemay be determined by, for example, the relative value or intensity ofthe corresponding measured optical property in the formation fluid. (Itis understood that the terms “first” and “second” need not be sequentialand only denote the order of addition of the fluids into the formationor the order of addition of zones treated in a formation. In otherwords, the first zone is merely penultimate to the second zone. Thus,for example, the “first zone” may refer to a third zone of a multi-zoneformation and the “second zone” may refer to a sixth zone of amulti-zone formation; the “first treatment fluid” may be a fourthtreatment fluid introduced while the “second treatment fluid” may be theeighth treatment fluid introduced.)

As one non-limiting example, referring to FIG. 1, a first group ofcarbon quantum dots exhibiting a first optical property may beintroduced into at least one of the first zone 101, the aquifer zone102, the second zone 103, the third zone 104, the fourth zone 105, thefifth zone 106, and the sixth zone 107 and at least a second group ofcarbon quantum dots exhibiting a second optical property may beintroduced into another of the first zone 101, the aquifer zone 102, thesecond zone 103, the third zone 104, the fourth zone 105, the fifth zone106, and the sixth zone 107. An absorption spectrum, an emissionspectrum or other optical property of produced fluids may be measured todetermine if any of the first group of carbon quantum dots or the secondgroup of carbon quantum dots are present in the produced fluid. Forexample, an emission spectrum of the produced fluid may be used todetermine a proportion of the produced fluid that originated from eachzone based on the fluorescence intensity of the carbon quantum dotsintroduced into each zone. By way of non-limiting example, carbonquantum dots introduced into a first zone with a first fracturing fluidmay be formulated to fluoresce at wavelengths that correspond to bluelight (e.g., at wavelengths of about 450 nm) and carbon quantum dotsintroduced into a second zone with a second fracturing fluid may beformulated to fluoresce at wavelengths that correspond to red light(e.g., at wavelengths of about 700 nm). An emission spectrum (e.g., afluorescence color) of produced fluid may indicate whether the producedfluid originated from the first zone or the second zone.

Referring to the carbon quantum dots here, in some embodiments, thefirst group of carbon quantum dots may include undoped carbon quantumdots and the at least a second group of carbon quantum dots may be dopedwith one or more of nitrogen, boron, silicon, and phosphorus. In otherembodiments, the first group of carbon quantum dots may be undoped ormay be doped with nitrogen, boron, silicon, or phosphorus and the atleast a second group of carbon quantum dots may be another of undoped ordoped with nitrogen, boron, silicon, or phosphorus.

In addition to monitoring different zones in hydrocarbon productionwells and determining the zone in which hydrocarbons have been producedfrom the formation, the carbon quantum dots may also be used to monitoroil and gas for flow assurance and for maintaining regulatorycompliance. The ability to analyze the fluids on-site, quickly andfrequently, further assists operators to detect flow assurance, assetintegrity and process problems early enabling them to take preventativeaction to minimize the risks of production loss and to adapt thetreatment operation.

Further, the carbon quantum dots may also be used to determine sites offlowback water and produced water as well as for detection or earlywarning of phenomena such as water breakthrough.

In addition to their use in hydraulic fracturing, the carbon quantumdots may be included in fluids used in well treating applications nearwellbore and may be directed toward improving wellbore productivityand/or controlling the production of formation sand. Particular examplesinclude gravel-packing and “frac-packs.”

In gravel-packing, sand is used to pre-pack a screen to prevent thepassage of formation particles or unconsolidated materials from theformation into the wellbore during production of fluids from theformation. Gravel-packing is essentially a technique for building atwo-stage filter downhole. The filter consists of gravel-pack sand and ascreen or liner. The gravel-pack sand is sized according to the particlesize distribution of the unconsolidated materials. The screen or linerhas openings that are sized to retain the gravel-pack sand. Thus thegravel-pack particulates retain the unconsolidated formation materialsand the screen or liner retains the gravel-pack particulates. Thegravel-pack particulates and the screen or liner act together to reduceor eliminate the production of the unconsolidated formation materialswith the oil or gas production. A slurry of sand introduced into thewell further may contain the carbon quantum dots. The slurry is thenpumped into the workstring within the well until the slurry is withinabout 150 to about 300 feet of the primary port. Positioning of acrossover service tool allows the slurry to travel into thescreen/casing annulus. Particulates are retained by the screen or linerand the remaining fluid leaks off into the formation allowing a tightlypacked sand filter to remain in place. Monitoring the carbon quantumdots provides information of the type and amount of the produced fluidfrom the formation.

The carbon quantum dots may further be used in a frac-pack operationwhere the unconsolidated formation is hydraulically fractured while atwo-stage filter of gravel-pack is simultaneously built. Infrac-packing, the processes of hydraulic fracturing and gravel-packingare combined into a single treatment to provide stimulated productionand an annular gravel-pack to reduce formation sand production.

Further, carbon quantum dots may be used in combination with an acid inan acid fracturing operation. Carbon quantum dots are stable in very lowpH also. CQDs can be mixed with acids for acid fracturing operations.The acid is a corrosive, very low pH acid which reacts with thesurrounding formation. The method is particularly effective withsandstone and carbonate formations. Acids such as hydrochloric acid,formic acid, and acetic acid are injected at high rates and pressuresinto the formation with the fluid to intentionally cause the formationto fail by inducing a fracture in the subterranean rock. In anotherembodiment, the fluid of the invention may contain the acid. Fractures,originating adjacent to the wellbore, initiate as two wings growing awayfrom the wellbore in opposite directions. The acid is used to dissolveor etch channels or grooves along the fracture face so that afterpressure is relieved and the fracture heals, there continues to existnon-uniform highly conductive channels, allowing unrestrainedhydrocarbon flow from the reservoir to the wellbore. In contrast, withpropped fracturing, fracture conductivity is maintained by propping openthe created fracture with a solid material, such as sand, bauxite,ceramic, and certain lighter weight materials. Conductivity in acidfracturing is obtained by etching of the fracture faces with an etchingacid instead of by using proppants to prevent the fracture from closing.Monitoring of the carbon quantum dots provides information of the typeand amount of the produced fluid from the formation and the success ofthe acid fracturing operation.

Carbon quantum dots may further be used, in addition to acid fracturing,in matrix acidizing. In matrix acidizing, a fluid containing an organicor inorganic acid or acid-forming material is injected into theformation below fracture pressure such that the acid or acid-formingmaterial reacts with minerals in the formation. A channel or wormholesis created within the formation. As subsequent fluid is pumped into theformation, it tends to flow along the channel, leaving the rest of theformation untreated. Matrix acidizing is often used to enhancenear-wellbore permeability. In addition to enhancing the production ofhydrocarbons, blockages caused by natural or man-made conditions mayfurther be removed during matrix acidizing. For instance, formationdamage caused by drilling mud invasion and clay migration may also beremoved during the process. The use of matrix acidizing is oftenpreferred in the treatment of carbonate formations since the reactionproducts are soluble in the spent acid. Monitoring of the carbon quantumdots during matrix acidizing informs the operator of the amount offluids being produced during the operation and further provides ameasurement on the value of the matrix acidizing operation.

In yet other embodiments, the carbon quantum dots may be used as atracer to determine fluid flow paths through the subterranean formationand into produced fluids. For instance, the carbon quantum dots may beintroduced into an injection fluids during at least one of waterflooding, steam assisted gravity drainage, steam flooding, cyclic steamstimulation, or other enhanced oil recovery stimulation processes.

In other embodiments, different carbon quantum dots are preferablyintroduced into the aqueous fluid introduced into the differentinjection wells. Fluids produced from one or more production wells maybe analyzed for the presence of the carbon quantum dots in the producedfluid. The presence of carbon quantum dots in produced fluids from aproduction well may indicate water breakthrough. Thus, not only canwater breakthrough in the production well be determined but theinjection well from which the water has flowed in into the productionwell can be identified. The injection well, into which the water in thebreakthrough water has been determined to have been initiallyintroduced, can be shut off. Thus, the carbon quantum dots can be usedto optimize enhancement of hydrocarbons during secondary recoveryoperations by shutting down the injection well and thus terminating theflow of water from the injection well directly into the production well.

The carbon quantum dots used in this embodiment are typically watersoluble. Carbon quantum dots are introduced into the aqueous fluid whichis then introduced into the injection well. The aqueous fluid introducedinto each of the injection wells contains qualitatively distinguishablecarbon quantum dots. The aqueous fluid serves to maintain pressure inthe hydrocarbon-bearing reservoir. The pressure is maintained above thebubble point. Should carbon quantum dots be detected in produced fluidfrom the production well, the operator would know to take remedialaction and shut down the injection well from which the carbon quantumdots had originally been introduced. The injection well, once shut down,may be repaired to prevent further flow of aqueous fluid into theproduction well.

EXAMPLES

The methods that may be described above or claimed herein and any othermethods which may fall within the scope of the appended claims can beperformed in any desired suitable order and are not necessarily limitedto any sequence described herein or as may be listed in the appendedclaims. Further, the methods of the present disclosure do notnecessarily require use of the particular embodiments shown anddescribed herein, but are equally applicable with any other suitablestructure, form and configuration of components.

While exemplary embodiments of the disclosure have been shown anddescribed, many variations, modifications and/or changes of the system,apparatus and methods of the present disclosure, such as in thecomponents, details of construction and operation, arrangement of partsand/or methods of use, are possible, contemplated by the patentapplicant(s), within the scope of the appended claims, and may be madeand used by one of ordinary skill in the art without departing from thespirit or teachings of the disclosure and scope of appended claims.Thus, all matter herein set forth or shown in the accompanying drawingsshould be interpreted as illustrative, and the scope of the disclosureand the appended claims should not be limited to the embodimentsdescribed and shown herein.

Embodiment 1

A system for determining at least one property of at least one fluid inat least one subterranean formation, the system comprising: a fluiddelivery system configured and positioned to deliver a fluid into atleast one of at least one subterranean formation and a wellboreextending through the at least one subterranean formation; a radiationsource within the wellbore, the radiation source configured to generateexcitation radiation; carbon quantum dots disposed in the fluid; and adetector within the wellbore, the detector configured to measure atleast one fluorescence property of the carbon quantum dots.

Embodiment 2

The system of Embodiment 1, wherein the carbon quantum dots compriseundoped carbon quantum dots.

Embodiment 3

The system of Embodiment 1, wherein the carbon quantum dots are dopedwith one or more of nitrogen, boron, silicon, and phosphorus.

Embodiment 4

The system of any one of Embodiments 1 through 3, wherein the carbonquantum dots comprise a first group of carbon quantum dots and at leasta second group of carbon quantum dots, the first group of carbon quantumdots formulated to exhibit a different fluorescence property than the atleast a second group of carbon quantum dots.

Embodiment 5

The system of Embodiment 4, wherein the first group of carbon quantumdots is dispersed in a first zone of the subterranean formation and theat least a second group of carbon dots is disposed in at least a secondzone of the subterranean formation.

Embodiment 6

The system of any one of Embodiments 1 through 5, wherein the radiationsource comprises a laser configured to generate excitation radiation ofa substantially monochromatic wavelength or a broadband radiation sourcecomprising one of a collimated LED, an uncollimated LED, or a whitelight.

Embodiment 7

The system of any one of Embodiments 1 through 6, wherein the carbonquantum dots include one or more of hydrophilic exposed surfaces andoleophilic exposed surfaces.

Embodiment 8

A system for determining at least one property of at least onesubterranean formation, the system comprising: at least one fiber opticcable within a wellbore extending through at least one subterraneanformation, the at least one fiber optic cable including at least oneoptical fiber comprising carbon quantum dots; a radiation source coupledto the at least one optical fiber, the radiation source configured togenerate excitation radiation for transmission through the at least oneoptical fiber; and a detector coupled to the at least one fiber opticcable, the detector configured to measure at least one fluorescenceproperty of the carbon quantum dots.

Embodiment 9

The system of Embodiment 8, wherein the carbon quantum dots are disposedwithin the at least one optical fiber.

Embodiment 10

The system of Embodiment 8, wherein the at least one optical fibercomprises a coating of the carbon quantum dots on at least a portionthereof.

Embodiment 11

The system of Embodiment 10, wherein the coating comprises a monolayerof the carbon quantum dots.

Embodiment 12

The system of any one of Embodiments 8 through 11, wherein the carbonquantum dots comprise a first group of undoped carbon quantum dots andat least a second group of carbon quantum dots doped with one or more ofnitrogen, boron, silicon, and phosphorus.

Embodiment 13

The system of any one of Embodiments 8 through 12, further comprising atleast another optical fiber coupled to the detector and configured totransmit emitted radiation from the carbon quantum dots to the detector.

Embodiment 14

The system of any one of Embodiments 8 through 11, or 13, wherein thecarbon quantum dots comprise one or more of nitrogen, boron, silicon,and phosphorus.

Embodiment 15

The system of any one of Embodiments 8 through 14, wherein the detectoris configured to measure at least one of an absorption spectrum, anemission spectrum, and a fluorescence intensity of the carbon quantumdots.

Embodiment 16

A method of forming carbon quantum dots, the method comprising:providing an electrolyte comprising a carbon source and a source of ionsto an electrochemical cell; introducing the electrolyte between platinumelectrodes of the electrochemical cell; and applying electrical currentbetween the platinum electrodes to form carbon quantum dots includingcarbon from the carbon source.

Embodiment 17

The method of Embodiment 16, wherein providing an electrolyte comprisinga carbon source comprises forming an electrolyte comprising one or moreof ethanol and ethanolamine.

Embodiment 18

The method of Embodiment 16 or Embodiment 17, further comprising formingthe electrolyte from one or more of a nitrogen source, a boron source, asilicon source, and a phosphorus source.

Embodiment 19

The method of any one of Embodiments 16 through 18, further comprisingforming at least some carbon quantum dots comprising one of nitrogen,boron, silicon, and phosphorus and at least some carbon quantum dotscomprising at least another of nitrogen, boron, silicon, and phosphorus.

Embodiment 20

The method of any one of Embodiments 16 through 19, further comprisingforming hydrophilic surfaces on the carbon quantum dots or formingoleophilic surfaces on the carbon quantum dots.

Embodiment 21

A method of determining at least one property within at least onesubterranean formation, the method comprising: introducing at least onefiber optic cable into at least one of at least one subterraneanformation and a wellbore extending into the at least one subterraneanformation; transmitting excitation radiation through the at least onefiber optic cable from a radiation source coupled to the at least onefiber optic cable; exposing carbon quantum dots disposed in a fluid inthe wellbore or on the at least one fiber optic cable to the excitationradiation; receiving, at an optical sensor coupled to the at least onefiber optic cable, an emitted radiation from the carbon quantum dotsresponsive to exposure of the carbon quantum dots to the excitationradiation; and measuring at least one of an emission spectrum and afluorescence intensity of the emitted radiation at a detector coupled tothe at least one fiber optic cable.

Embodiment 22

The method of Embodiment 21, wherein measuring at least one of anemission spectrum and a fluorescence intensity of the emitted radiationat a detector comprises measuring the at least one of an emissionspectrum and the fluorescence intensity after a time delay.

Embodiment 23

The method of Embodiment 21 or Embodiment 22, wherein introducing atleast one fiber optic cable into at least one of at least onesubterranean formation and a wellbore comprises introducing at least onefiber optic cable comprising at least one surface coated with carbonquantum dots into at least one of the at least one subterraneanformation and the wellbore.

Embodiment 24

The method of any one of Embodiments 21 through 23, further comprisingdisposing a fluid comprising carbon quantum dots doped with one or moreof nitrogen, boron, and phosphorus into at least one of the at least onesubterranean formation and the wellbore.

Embodiment 25

The method of any one of Embodiments 21 through 23, further comprisingintroducing carbon quantum dots exhibiting a first optical property intoa first zone of the at least one subterranean formation and introducingcarbon quantum dots having a second optical property into a second zoneof the at least one subterranean formation.

Embodiment 26

A method of enhancing the productivity of hydrocarbon containing fluidsfrom a subterranean formation penetrated by a well comprising (a)pumping into the well a fluid comprising a tracer which is eitherhydrocarbon soluble, water soluble or both hydrocarbon soluble and watersoluble and further wherein the tracer comprises carbon quantum dots;and (b) identifying the tracer in fluids produced from the well.

Embodiment 27

The method of Embodiment 26, wherein the carbon quantum dots areundoped.

Embodiment 28

The method of Embodiment 26 or 27, wherein at least a portion of thesurface of the carbon quantum dots contain carboxyl groups, hydroxylgroups and/or ether groups.

Embodiment 29

The method of Embodiment 26, wherein the carbon quantum dots are dopedwith one or more of nitrogen, boron, silicon, and phosphorus.

Embodiment 30

The method of Embodiment 26, wherein at least a portion of the surfaceof the carbon quantum dots are hydrophilic and/or oleophilic.

Embodiment 31

The method of any of Embodiments 26 to 30, wherein the fluid is pumpedinto the well at a pressure sufficient to enlarge or create a fracturein the formation and further wherein a fracture is created or enlargedin the subterranean formation.

Embodiment 32

The method of any of Embodiments 26 to 31, wherein the carbon quantumdots comprise a first group of carbon quantum dots and at least a secondgroup of carbon quantum dots, the first group of carbon quantum dotsformulated to exhibit a different detection property than the at least asecond group of carbon quantum dots.

Embodiment 33

The method of Embodiment 32, wherein at least one group of carbonquantum dots is soluble in water and at least one other group of carbonquantum dots is soluble in hydrocarbon.

Embodiment 34

The method of Embodiment 32, wherein the first group of carbon quantumdots is dispersed in a first zone of the subterranean formation and theat least a second group of carbon dots is disposed in at least a secondzone of the subterranean formation.

Embodiment 35

The method of any of Embodiments 26 to 32, wherein the fluid is pumpedinto the well during a sand control operation.

Embodiment 36

The method of Embodiment 35, wherein the formation is an unconsolidatedformation and further wherein the sand control operation issimultaneously conducted while the unconsolidated formation ishydraulically fractured.

Embodiment 37

A method of fracturing multiple productive zones of a subterraneanformation penetrated by a well, the method comprising: (a) pumpingfracturing fluid into the multiple productive zones at a pressuresufficient to enlarge or create fractures in the multiple productivezones, wherein the fracturing fluid pumped into the multiple productivezones comprises fluorescent carbon quantum dots which are eitherhydrocarbon soluble, water soluble or both hydrocarbon soluble and watersoluble and further wherein the fracturing fluid pumped into each of themultiple productive zones contains different fluorescent carbon quantumdots; (b) recovering fluid containing hydrocarbons from one or more ofthe multiple productive zones; (c) detecting the fluorescent carbonquantum dots in the recovered fluid; and (d) identifying the zone fromwhich the recovered fluid was produced from the fluorescent carbonquantum dots.

Embodiment 38

The method of Embodiment 37, further comprising quantitativelydetermining the amount of hydrocarbons produced from the identified zonefrom the fluorescent carbon quantum dots.

Embodiment 39

The method of Embodiment 38, wherein the fluorescent carbon quantum dotsinclude one or more of hydrophilic exposed surfaces and/or oleophilicexposed surfaces.

Embodiment 40

The method of Embodiment 39, wherein at least a portion of the surfaceof the fluorescent carbon quantum dots contains carboxyl groups,hydroxyl groups and/or ether groups.

Embodiment 41

The method of Embodiment 37, wherein the at least a portion of thecarbon quantum dots are doped with one or more of nitrogen, boron,silicon, and phosphorus.

Embodiment 42

A method of fracturing multiple zones of a subterranean formationpenetrated by a well which comprises: (a) pumping into each zone of theformation to be fractured a fracturing fluid, wherein the fracturingfluid pumped into each zone comprises a qualitatively distinguishabletracer comprising carbon quantum dots which are either hydrocarbonsoluble, water soluble or both hydrocarbon soluble and water soluble;(b) enlarging or creating a fracture in the formation; (c) recoveringfluid from at least one of the multiple zones; and (d) identifying thezone within the subterranean formation from which the recovered fluidwas produced by identifying the carbon quantum dots in the recoveredfluid.

Embodiment 43

The method of Embodiment 42, further comprising determining the amountof the recovered fluid from the identified zone of step (d).

Embodiment 44

The method of Embodiment 42, wherein at least a portion of the surfaceof the carbon quantum dots are hydrophilic and/or oleophilic.

Embodiment 45

The method of Embodiment 44, wherein at least a portion of the surfaceof the carbon quantum dots contain carboxyl groups, hydroxyl groupsand/or ether groups.

Embodiment 46

The method of Embodiment 42, wherein the carbon quantum dots are dopedwith one or more of nitrogen, boron, silicon, and phosphorus.

Embodiment 47

A method of monitoring the production of fluids produced in multipleproductive zones of a subterranean formation penetrated by a well, themethod comprising: (a) pumping fracturing fluid into the multipleproductive zones at a pressure sufficient to enlarge or create fracturesin each of the multiple productive zones, wherein the fracturing fluidcomprises fluorescent carbon quantum dots which are either hydrocarbonsoluble, water soluble or both hydrocarbon soluble and water soluble andfurther wherein the fluorescent carbon quantum dots pumped into each ofthe multiple productive zones is qualitatively and/or quantitativelydistinguishable; and (b) monitoring the amount of fluids produced fromat least one of the multiple productive zones from the carbon quantumdots in the produced fluid.

Embodiment 48

The method of Embodiment 47, wherein the monitoring is in real time.

Embodiment 49

The method of either Embodiments 47 or 48, wherein the monitoring isconducted on the fly.

Embodiment 50

The method of any one of Embodiments 47 to 49, further comprisingdetermining the presence of dispersed oil in produced water from thecarbon quantum dots.

Embodiment 51

The method of Embodiment 47, wherein at least a portion of the surfaceof the carbon quantum dots are hydrophilic and/or oleophilic.

Embodiment 52

The method of Embodiment 51, wherein at least a portion of the surfaceof the carbon quantum dots contains carboxyl groups, hydroxyl groupsand/or ether groups.

Embodiment 53

The method of Embodiment 51, wherein the carbon quantum dots are dopedwith one or more of nitrogen, boron, silicon, and phosphorus.

Embodiment 54

A method for enhancing the production of hydrocarbons from a productionwell penetrating a hydrocarbon-bearing formation, wherein one or moreinjector wells are associated with the production well, the methodcomprising: (a) introducing into one or more of the injector wells anaqueous fluid comprising fluorescent carbon quantum dots; (b) flowing atleast a portion of the aqueous fluid comprising the fluorescent carbonquantum dots from the one or more injector wells into the productionwell; and (c) recovering hydrocarbons from the production well.

Embodiment 55

The method of Embodiment 54, wherein at least a portion of the surfaceof the carbon quantum dots are hydrophilic and/or oleophilic.

Embodiment 56

A method for determining water breakthrough in a production wellassociated with one or more injector wells, the method comprising: (a)injecting an aqueous fluid comprising fluorescent carbon quantum dots astracer into an injector well; (b) flowing the aqueous fluid from theinjector well into the production well; (c) producing fluid from theproduction well; (d) determining water breakthrough in the productionwell by qualitatively determining the presence or quantitativelymeasuring the amount of the fluorescent carbon quantum dots in theproduced fluid.

Embodiment 57

The method of Embodiment 56, wherein at least a portion of the surfaceof the carbon quantum dots are hydrophilic and/or oleophilic.

Embodiment 58

A method of increasing hydrocarbon production from a production wellpenetrating a hydrocarbon-bearing reservoir, wherein more than oneinjection well is associated with the production well, the methodcomprising: (a) injecting an aqueous fluid having a water-soluble tracercomprising carbon quantum dots into the more than one injection well andmaintaining pressure in the hydrocarbon-bearing reservoir above thebubble point of the hydrocarbons in the reservoir, wherein the aqueousfluid pumped into each of the injection wells contains qualitativelydistinguishable carbon quantum dots; (b) identifying from hydrocarbonsrecovered from the production well, upon water breakthrough in theproduction well, the injection well into which the breakthrough waterwas injected by qualitatively determining the presence of the carbonquantum dots in the recovered hydrocarbons; and (c) shutting off theinjector well identified in step (b).

Although the foregoing description contains many specifics, these arenot to be construed as limiting the scope of the disclosure, but merelyas providing certain embodiments. Similarly, other embodiments may bedevised that do not depart from the scope of the disclosure. Forexample, features described herein with reference to one embodiment alsomay be provided in others of the embodiments described herein. The scopeof the disclosure is, therefore, indicated and limited only by theappended claims and their legal equivalents, rather than by theforegoing description. All additions, deletions, and modifications toembodiments of the disclosure, as described and illustrated herein,which fall within the meaning and scope of the claims, are encompassedby the disclosure.

What is claimed is:
 1. A method of determining a pH of a wellbore fluidwithin a wellbore in communication with a subterranean formation, themethod comprising: introducing carbon quantum dots comprising a carboncore into a wellbore fluid; exposing the wellbore fluid to radiationfrom an electromagnetic radiation source; and measuring at least onefluorescence property of the carbon quantum dots within the wellborefluid to determine a pH of the wellbore fluid.
 2. The method of claim 1,wherein introducing carbon quantum dots into a wellbore fluid comprisesintroducing carbon quantum dots comprising at least one of nitrogen,boron, silicon, and phosphorus into the wellbore fluid.
 3. The method ofclaim 1, wherein introducing carbon quantum dots into a wellbore fluidcomprises introducing carbon quantum dots having a diameter betweenabout 1 nm to about 10 nm into the wellbore fluid.
 4. The method ofclaim 1, wherein introducing carbon quantum dots into a wellbore fluidcomprises introducing at least one optical fiber comprising the carbonquantum dots into the wellbore.
 5. The method of claim 1, whereinintroducing carbon quantum dots into a wellbore fluid comprisesintroducing a fluid comprising carbon quantum dots into the wellbore. 6.The method of claim 1, wherein measuring at least one fluorescenceproperty of the carbon quantum dots within the wellbore fluid comprisesmeasuring a fluorescence intensity of the carbon quantum dots responsiveto exposure to electromagnetic radiation from the electromagneticradiation source.
 7. The method of claim 1, wherein measuring at leastone fluorescence property of the carbon quantum dots within the wellborefluid comprises measuring the at least one fluorescence property of thewellbore fluid while the wellbore fluid is in the subterraneanformation.
 8. The method of claim 1, wherein measuring at least onefluorescence property of the carbon quantum dots within the wellborefluid comprises measuring the at least one fluorescence property of thecarbon quantum dots with a detector located within the subterraneanformation.
 9. The method of claim 1, wherein introducing carbon quantumdots into a wellbore fluid comprises introducing carbon quantum dotsformulated to fluoresce within the ultraviolet spectrum, within thevisible spectrum, or within the infrared spectrum.
 10. The method ofclaim 1, wherein introducing carbon quantum dots into a wellbore fluidcomprises introducing carbon quantum dots formulated to exhibitupconversion properties into the wellbore fluid.
 11. A method ofcontinuously measuring a pH of a fluid within a subterranean formation,the method comprising: introducing carbon quantum dots comprising acarbon core into a fluid in communication with a subterranean formation;exposing the carbon quantum dots to electromagnetic radiation from anelectromagnetic radiation source; and measuring at least onefluorescence property of the carbon quantum dots responsive to exposureto the electromagnetic radiation while the carbon quantum dots arelocated within the fluid in the subterranean formation.
 12. The methodof claim 11, further comprising selecting the carbon quantum dots tocomprise a first group of carbon quantum dots doped with at least one ofnitrogen, boron, silicon, and phosphorus and a second group of carbonquantum dots comprising undoped carbon quantum dots.
 13. The method ofclaim 11, wherein measuring at least one fluorescence property of thecarbon quantum dots responsive to exposure to the electromagneticradiation comprises measuring at least one of an adsorption spectrum, anemission spectrum, an absorption intensity, a peak absorptionwavelength, a peak emission wavelength, and a fluorescence intensity ofthe carbon quantum dots.
 14. The method of claim 11, wherein introducingcarbon quantum dots into a fluid in communication with a subterraneanformation comprises introducing the carbon quantum dots into thesubterranean formation with a fiber optic cable.
 15. The method of claim11, wherein exposing the carbon quantum dots to electromagneticradiation comprises exposing the carbon quantum dots to a substantiallymonochromatic wavelength.
 16. The method of claim 11, further comprisingproviding the electromagnetic radiation source within the subterraneanformation.
 17. A method of determining a pH of a fluid within a wellboreextending through a subterranean formation, the method comprising:introducing a fiber optic cable comprising at least one optical fibercomprising carbon quantum dots having a carbon core into a wellbore;exposing the carbon quantum dots to electromagnetic radiation while thecarbon quantum dots are exposed to a fluid in the wellbore; andmeasuring at least one fluorescence property of the carbon quantum dotsto determine a pH of the fluid in the wellbore.
 18. The method of claim17, wherein introducing a fiber optic cable comprising at least oneoptical fiber comprising carbon quantum dots into a wellbore comprisesintroducing a fiber optic cable comprising a coating of carbon quantumdots into the wellbore.
 19. The method of claim 17, wherein introducinga fiber optic cable comprising at least one optical fiber comprisingcarbon quantum dots into a wellbore comprises introducing a fiber opticcable comprising carbon quantum dots dispersed in at least a portion ofthe fiber optic cable.
 20. The method of claim 17, wherein measuring atleast one fluorescence property of the carbon quantum dots comprisesmeasuring electromagnetic radiation emitted by the carbon quantum dotswith a detector coupled to the at least one optical fiber.